1. Field of the Invention
This invention relates to a sensor for detecting the presence of gases in air, and more specifically to a sensor having a multilayered structure for adhering a catalyst over a resistive sensing element.
2. Description of the Related Art
The use of catalysts to enhance the rate of chemical reactions is well known. A catalyst can be any substance that affects a chemical reaction rate without itself being consumed or undergoing a chemical change in the process. Catalysts may be inorganic, organic, or a complex composition of organic groups and metal halides.
The present invention is directed at the use of a catalyst in gas sensors. Catalytic gas sensors function by creating a chemical reaction when the gas to be sensed comes in contact with the catalyst. Often, the chemical reaction creates a temperature change that can be used to effect the electrical resistance of a conductor. Thus, typical sensing elements are conductors that both exhibit conductivity changes as the temperature varies and are coated with a catalyst.
Typically, gas sensing devices use both a catalyzed sensor and a non-catalyzed reference sensor. The two sensors are typically coupled in a Wheatstone bridge arrangement. Gas concentrations can be determined by measuring the difference in voltage drop across the catalyzed and non-catalyzed coated conductors.
3. Related Art
Examples of patents that are related to the present invention are as follows, and each patent is herein incorporated by reference for the supporting teachings:
U.S. Pat. No. 4,045,177, is an apparatus for detecting combustible gases.
U.S. Pat. No. 4,322,383, is a gas component detection device composed of two metal oxide sensors.
U.S. Pat. No. 4,447,397, is a catalytic gas sensor having a filament coated with titanium dioxide (TiO.sub.2).
U.S. Pat. No. 4,957,705, is an oxygen gas concentration detecting device.
U.S. Pat. No. 5,445,796, is an oxygen concentrating sensor with a heat resistant coating.
U.S. Pat. No. Re. 33,980, is a thick film gas sensitive element.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singularly or when considered in combination, applicant's claimed invention.
4. Problems with Related Art
A problem with current gas sensors is that there is no compact, cost effective, and durable hydrocarbon or nitrogen oxide sensors that is suitable for functioning in the harsh environment of an automobile exhaust system. Sensors are needed to measure these gases to assure that automobiles are complying with emission requirements. The auto industry currently uses oxygen sensors to indirectly measure hydrocarbons and nitrogen oxides, but indirect measurement is not as accurate as more direct measurement.
A further problem is the difficulty in designing a sensor structure that can operate properly when exposed to temperature extremes, continuous vibrations, mechanical shock and contaminants without experiencing a significant degradation of performance over its expected life.
It is noted that the above described problems, and other problems are solved through the subject invention and will become more apparent to one skilled in the art, from the detailed description of the subject invention.